An electric power steering apparatus of an automobile uses a speed reduction gear. A column type EPS, for example, reduces the speed of rotation of an electric motor and increases output power in the course of transmitting power from a small gear such as a worm gear to a large gear such as a worm wheel in a speed reduction gear, and imparts the increased output power to the column thereby assisting the steering operation with amplified torque.
It is necessary to allow an appropriate amount of backlash in the engaging of a small gear and a large gear to function as a speed reducing mechanism. However, when the direction of rotation of a gear is reversed or when the reaction force of the road to the tire is transmitted to the gear during travel on a rough road such as stone-paved road, backlash may cause a sound of striking gear teeth which, when transmitted into the cabin as a noise, may cause the driver to feel uneasy.
For this reason, the speed reduction gear has been conventionally built by so-called by-layer assembly in which a small gear and a large gear are selected to form such a combination that has optimum backlash. However, such a method of assembly leads to a significantly low productivity. In addition, when the by-layer assembly is used, there remains another problem that the steering torque fluctuates due to eccentricity in the alignment of worm wheel shaft. Such problems as described above are not limited to the speed reduction gear of an electric power steering apparatus, and are encountered in speed reduction gears in general that comprise small gears and large gears.
Accordingly, it has been proposed for the speed reduction gear of an electric power steering apparatus, to enable the worm shaft to be shifted toward the worm wheel and provide urging means such as a spring for urging the worm shaft in the shifted direction, thereby to eliminate backlash (see, for example, Japanese Unexamined Patent Publication No. 2000-43739).
However, the speed reduction gear described above has very complicated structure, which leads to increasing manufacturing cost.